What is bootstrapping The process of loading larger programs for execution than can fit within a single disk sector Serious Pentium explorations Experimenting with most Pentium features will require us to write larger size demos than can fit in one disk sector 512 bytes So we need a way to load such programs into memory when no OS is yet running And we need a convenient way to place such programs onto a persistent storage medium so they can easily be accessed Our classroom setup Our workstations hard disks have been partitioned in way that provides a large unused storage area for us to use freely But other portions of these hard disks are dedicated to supporting vital courseware for students who are taking other classes We have to understand how to access our free area without disrupting anyone else Fixed Size blocks All data transfers to and from the hard disk are comprised of fixed size blocks called sectors whose size equals 512 bytes On modern hard disks these sectors are identified by sector numbers starting at 0 This scheme for addressing disk sectors is known as Logical Block Addressing LBA So the hard disk is just an array of sectors Visualizing the hard disk A large array of 512 byte disk sectors 0 1 2 3 Disk storage capacity in bytes total number of sectors x 512 bytes sector Example If disk capacity is 160 GigaBytes then the total number of disk sectors can be found by division 160000000000 bytes 512 bytes per sector assuming that you have a pocket calculator capable of displaying enough digits Disk Partitions The total storage area of the hard disk is usually subdivided into non overlapping regions called disk partitions Partition 1 Partition 2 Partition 3 unused Master Boot Record A small area at the beginning of the disk is dedicated to managing the disk partitions 0 1 2 MBR partition 1 In particular sector number 0 is known as the Master Boot Record very important Format of the MBR The MBR is subdivided into three areas The boot loader program e g GRUB The partition table data structure The MBR signature i e 0x55 0xAA 512 bytes Boot Loader 446 bytes Partition Table 64 bytes signature 2 bytes Reading the MBR To see the hard disk s Partition Table we must read the entire Master Boot Record We ignore the boot loader and signature But we will need to understand the format of the data stored in that Partition Table We first need to know how to devise code that can transfer the MBR sector 0 from the hard disk into a suitable memory area Partition Table Entries The MBR is an array containing four datastructures called partition table entries 16 bytes S T A T U S T Y P E Starting sector ID number Partition length in sectors Some fields contain obsolete information TYPE ID Each partition table entry has a TYPE ID TYPE ID is 0x07 for a Windows partition TYPE ID is 0x83 for our Linux partition TYPE ID is 0x00 when the entry is unused You can find a list of TYPE ID numbers posted on the internet see our website Our disks have an extra Linux partition that nobody else is using this semester BIOS Disk Drive Services An assortment of disk access functions is available under software Interrupt 0x13 Originally there were just six functions to support IBM PC floppy diskette systems More functions were added when PC XTs introduced the use of small Hard Disks Now with huge hard disk capacities there is a set of Enhanced Disk Drive services Phoenix Technologies Ltd You can find online documentation for the BIOS EDD specification 3 0 see website We ll use function 0x42 to read the MBR It requires initializing some fields in a small data structure the Disk Address Packet Then we load parameters in four registers DS SI address of the DAP DL disk ID and AH 0x42 and execute int 0x13 EDD Disk Address Packet 7 6 5 4 segment address offset address of transfer area of transfer area 3 reserved 0x00 2 sector count 1 reserved 0x00 Logical Block Address of disk sector 64 bits Physical address of memory transfer area 64 bits in case segment offset above is 0xFFFF FFFF 0 packet length The MBR parameters Here are assembly language statements that you could use to create a Disk Address Packet for reading the hard disk s Master Boot Record into the memory area immediately following the 512 byte BOOT LOCN area packet byte 16 0 packet size 16 bytes byte 1 0 sector count 1 sector word 0x0200 0x07C0 transfer area s address quad 0 MBR s Logical Block Address Our demo program named finalpte s uses statements similar to these How we search the Partition Table Our demo program finalpte s locates the final valid entry in the MBR Partition Table It displays the contents of that entry i e four longwords in hexadecimal format To do its search it simply scans the table entries in backward order looking for the first entry that has a nonzero type code Instructions we could use mov mov 0x03FE si 4 cx point DS SI to signature word setup count of table s entries sub cmpb loope 16 si 0x00 4 si nxpte back up to the previous entry entry s type code is defined no examine the next entry jcxz nopte search fails if CX reached 0 nxpte If we get here then DS SI is pointing to the final valid PT entry jmp found nopte We should never arrive here unless no valid partitions exist Extended partitions The hard disk s Master Boot Record only has room for four Partition Table Entries But some systems need to use more than four disk partitions so a way was devised to allow one of the MBR s partition table entries to describe an extended partition This scheme does not seem to have been standardized yet hence confusion The Linux scheme Partition 5 Partition 1 Partition 6 Partition 2 Partition 3 Extended Partition partition 4 Partition 7 Our cs686ipl s boot loader We created a boot loader that will work with Linux systems that have extended disk partitions Initial Program Loader It uses the EDD Read Sectors function and it will read 127 consecutive sectors from the disk s final Linux type partition It transfers these disk sectors into the memory arena at address 0x00010000 Our controls s demo program To demonstrate our boot loader we wrote a short program that can be loaded into memory at 0x10000 and then executed It will display some useful information and thereby verify that our boot loader worked Our boot loader requires that a special program signature i e 0xABCD must occupy the first word of any program it attempts to execute as a sanity check Depiction of boot strapping We install our cs686ipl b loader into the boot sector of disk